Helical compression springs or compression coil springs are used in a wide range of applications owing to the favorable spring properties of the springs of this type. A coil end often includes a closely wound (or closed) turn so that the coil end provides a uniform contact surface. Grinding the end surface of a coil spring is also known as a means for providing a uniform contact surface. However, the entire contact surface of the closely wound turn and ground end may not contact an opposing member, such as a spring seat, but instead, they tend to contact the opposing member partially at unpredictable points, which can result in a varying spring property or varying relationship between the load and the compression stroke. Also the closely wound turn and ground end are not effective in producing the spring force. Therefore, in applications where the length of the coil spring is desired to be minimized while providing a maximum spring action, an open end coil spring is sometimes used. The last coil at either end of such a coil spring may be an open or closed turn, and may, or may not be, ground. Such a coil may be referred to as the “end turn”.
Now, taking an open and unground end coil as example, contact between a coil end and a spring seat is described in detail hereinafter. Because the coil spring having an open end does not sit on a flat surface in a stable manner, a spring seat having a contoured support surface is sometimes used. In either case, the state of contact inevitably changes as the coil spring is placed under a load and compressed without regard to whether the spring seat is contoured or not. More specifically, when the spring load is light, the contact between the coil wire of the end turn and the spring seat takes place at a point or a short length of the coil wire. However, as the spring load increases the length of contact between the coil wire of the end turn and the spring seat progressively and continually increases along the length of the coil wire. In addition, the contacting region can vary significantly due to different specifics of individual coil springs.
This means that the centroid of the contact pressure between the coil spring and spring seat can vary depending on the spring load. This in turn causes a change in the line of action of the spring force. Also, because the part of the coil wire which is in contact with the spring seat is not capable of any further deflection, and is, therefore, ineffective in increasing the spring force, an increase in the contacting part between the coil wire and the spring seat resulting from an increase in the spring load means a decrease in the number of effective turns of the coil spring. In many applications, such changes in the spring property with the change in the spring load are not desirable.
For instance, in a strut wheel suspension system for motor vehicles, because the point of input of the force which pushes up the tire does not agree with the axial line of the linear hydraulic damper due to the problems associated with the vehicle body structure and the convenience of assembly work, as the tire moves vertically, a certain bending moment is produced in the damper. This bending moment increases the frictional resistance between the damper tube and the piston and between the piston rod and rod guide, and hampers the smooth telescopic motion of the damper. This not only impairs the ride quality but also adversely affects the durability of the sliding parts.
Therefore, in a strut wheel suspension system, it is a common practice to offset the spring force axis of the coil spring from the central axial line of the damper in such a manner that a bending moment is produced in the coil spring that cancels the bending moment acting on the damper as the tire moves vertically. It is therefore important to control the line of action of the spring force under all conditions.
However, when an open-end coil spring C is used in the strut wheel suspension system as shown in FIG. 10, the length of the coil wire at which the end turn 1 of the coil spring C contacts the spring seat 3 progressively increases as the compressive stroke of the coil spring increases as illustrated in FIGS. 11A to 11C, and the number of turns of the coil spring which can deflect, or the number of effective turns of the coil spring, decreases with the increase in the spring load.
The decrease in the number of effective turns increases the spring constant. Therefore, the conventional coil spring demonstrates a nonlinear deflection property as represented by a curve in the graph of FIG. 12, instead of a linear relationship between the load and compressive stroke. In addition, because individual coil springs may have different specifics, the decrease in the number of effective turns may occur in different fashions for different coil springs, leading to varying compression property.
Also, because the centroid of contact pressure between the spring seat and the end turn of the coil spring varies depending on the magnitude of the load, the axis of the spring force changes in angle with the vertical motion of the tire, and the canceling action of the coil spring with respect to the bending moment varies for each individual coil spring. This may result in a coil spring which does not contribute to the action of improving the ride quality.
Furthermore, the spring property of the coil spring is required to be properly tuned so that the wheel suspension system may demonstrate an optimum performance. However, if the spring property varies depending on the spring load or the compression stroke, any exact tuning becomes impossible. The spring may demonstrate an optimum property at a certain load, but does not do so at different load conditions. This is a problem not only for the springs of wheel suspension systems but also for other applications where any tuning of the spring property is required.